Many institutions, such as banks and airlines, allow customers to access over the telephone a wide variety of services and account information. Before the advent of touch-tone telephones, a customer would obtain these services and information through interacting with a live operator. As touch-tone telephones became more prevalent in homes, these institutions began switching to automated customer-access systems. After dialing a telephone number, a customer using such a system would be asked to enter an identifier or confirmation number. An identifier typically comprises a string of alphanumeric characters.
When using touch-tone phones with an automated customer-access system, a user can enter an identifier by sequentially pressing a series of keys provided on the telephone keypad. Each pressed key corresponds to a different character in the identifier. The pressing of these keys produces a series of tones that are provided over a telephone network to the institution. At the institution, the series of tones are decoded to produce the entered identifier, and if the identifier entered by the user is determined to correspond to a valid identifier, then the user is allowed to enter commands, again through the telephone keypad, that provide access to whatever services are offered by the institution.
Recently, automated customer-access systems have eliminated the use of telephone keypads to obtain the identity of a valid user. Instead of entering an identifier through a telephone keypad, a user is prompted to speak the identifier into the telephone handset. For example, the user may speak into the telephone the identifier "JB123E". The user's voice signal is transmitted over the phone lines to the institution, which employs a speech recognition system to produce a recognized identifier that is intended to correspond exactly to the identifier spoken by the user.
However, such exact correspondence is difficult to attain due to, for example, the deterioration of voice signals that routinely occurs over conventional telephone lines, and the different ways the same words are pronounced by different users. Because of these problems, the speech recognizer may confuse similar sounding letters and numbers. Thus, a speech recognizer may confuse the letter "A" with the number "8", the letter "K", or the letter "J". Similarly, the speech recognizer may confuse the letter "C" with the letter "D" or the number "3". For example, given that a user speaks the identifier "JB123E" into a telephone, the speech recognizer may produce "AE123D" as an output.
One known way to increase the accuracy of known speech recognition systems is to append a checksum onto the identifier. For example, U.S. Pat. No. 5,222,187 discloses the use of a checksum added to an identifier to improve the accuracy of a speech recognition system. The checksum provides a way to independently verify that the other alphanumeric characters of the identifier were correctly recognized. Numerical values assigned to each alphanumeric character are processed by a predetermined checksum operation. The resultant number should equal the checksum identified by the speech recognition system. If it does not, there is an indication that an error has occurred.
However, one drawback with using a checksum is that an additional character (i.e., the checksum) must be added to the identifier. This additional character must be recognized by the speech recognition system. This by itself increases the chances of a recognition error because the accuracy of a speech recognition system decreases as the number of characters in an identifier required to be recognized are increased.
Based on the foregoing, there is a need for a speech recognition system that uses a checksum to recognize an identifier but does not require any additional characters to be added to the identifier.